Non-HDL cholesterol
Everything except HDL: all the cholesterol carried by particles that can lodge in artery walls.
Non-HDL cholesterol is total cholesterol minus HDL. What remains is the cholesterol carried by LDL and the other apoB-containing particles, which are the ones capable of entering and staying in an artery wall.
It matters because it captures the whole atherogenic group rather than LDL alone, and because it needs no fasting sample and no calculation that breaks down when triglycerides are high, which is a known weakness of the usual LDL estimate.
In practice UK guidance has moved towards non-HDL cholesterol as a primary lipid target, and it tracks closely with ApoB, which counts the same particles directly.